Space charge characterization for the 21th century

Various methods of characterizing insulating materials by their ability to take up charge, retain it, and release it, are reviewed critically in search of measurable quantities that could be used to predict material behavior under stress up to failure conditions. Space charge characterization data on different types of materials from polymers to inorganic single crystals and ceramics are surveyed. The charging behavior is found to be influenced by many details such as surface condition and residual stresses. The traditional approach of linking dielectric breakdown to an intrinsic critical field for the material is tested against the newly emerging view that breakdown could be linked to space charge trapping at defect sites and to the attendant energetics of the mechanically strained lattice. The characterization process thus requires more care than was previously thought necessary, but after more research should become more predictive     

Influence of Stoichiometry on the Electrical and Mechanical Behavior of Yttrium Oxide Ceramics

Because of their method of preparation, ceramics contain point defects, the concentration of which may vary from the center to the boundary of the grains, which may change the bulk properties of the material. The electronic structure of the oxygen-related defects has been studied by electron spectroscopy in yttria ceramics submitted to various heating conditions under controlled atmospheres. The differences in the mechanical and electrical behaviors are observed to correlate with the defect content.     

Characterization of non‐conducting materials: a promise of quality and productivity improvements in various industrial fields

An increase in reliability and a decrease in production cost of systems that include insulators require a better knowledge of the origins of friction, wear, adhesion, fracture and electric breakdown. By doing this, for example, a manufacturer could adapt his fabrication to the user's operational conditions whereas, at present, he only can offer a catalogue of standard specified products. The macroscopic behaviour of a material depends on the defects therein; these in turn determine the electric charge distribution and the internal energy of the material. Seen in this light, many pioneering results can be understood and used to clarify the experiments required for validating new theories of breakdown or wear. Several laboratories have worked together to demonstrate the direct relationships that exist between the microscopic parameters of charge and energy localization or relaxation and macroscopic behaviour. One essential point was to find how to carry out experiments whilst preserving the material's intrinsic defect state during sample preparation and how to characterize defects related to stresses applied to the material. Another point was to investigate the phenomena that occur at the early stage of wear, fracture and breakdown before catastrophic failure. Therefore the use of time‐ and spatially‐resolved measurements has been essential. This work reviews some pioneering results, pointing out the importance of polarization and conduction on apparently different behaviour (wear, adhesion and breakdown). These results have been explained from classical solid state physics and electrostatics backgrounds. When the experiments were carried out, however, characterization techniques for interpretating the results were missing, so that they did not receive the attention they deserved. Surprisingly, and as discussed in the second part, because these new characterization techniques have not been sufficiently disseminated in industry, these pioneering results and theories have not been practically applied. As a consequence, standard insulator specifications still do not reflect our actual knowledge. In the third part, some examples illustrate technological progress that can be made by combining some new characterization techniques with the standard ones. © 2001 Society of Chemical Industry     

Effect of Nonstoichiometry on Fracture Toughness and Hardness of Yttrium Oxide Ceramics

This work deals with the changes in mechanical properties of yttrium oxide ceramics induced by nonstoichiometry. The maximum fracture toughness, K_lc, is observed at the stoichiometric composition. For an oxygendeficient ceramic, a decrease of K_lc is observed from 3.5 to 2.3 MPa.m^1/2. On the other hand, the Vickers hardness seems to be less dependent on stoichiometry. These results are discussed in the frame of the evolution of the Y-O bond with the stoichiometry. They set in particular the problem of the role of electrostatic energy stored in a brittle material containing charged defects in the energy balance controlling crack propagation.     

Adherence of Lubricant Coatings to Porous Materials

The porosity which is used as a starting point for the present study is the porosity acquired by a sulfuration treatment in a melted salts bath, on a soft steel surface (0.1 percent carbon). Examination of the porosity of the surface and subsurface layers is made, using a scanning electron microscope. Communicating pores (open porosity) in the range 0.2 to 2 microns can be seen.

Partial filling with PTFE is obtained by vaporizing an adequate aerosol and subsequent heating of the sample. The degree of penetration of the polymer is assessed with the help of the same scanning electron microscope, having an X-ray analyser incorporated.

Very interesting tribological properties have been communicated in this way to the surface and subsurface layers: they are not only antiseizure, but they can creep and deform under very high friction pressure without any modification in their metallurgical structure, the PTFE film remaining unchanged.     

Nonstoichiometry in Pure and Zr-Doped Yttria Ceramics: An EXAFS and XPS Study

Nonstoichiometry is exhibited by many ceramics at very high temperatures. The high-temperature treatment of yttria, in particular, can induce a large concentration of point defects that changes the electrical and mechanical behavior of the solid. In the present study, the electronic and geometric structures of nonstoichiometric pure and Zr-doped yttria are examined by X-ray photoelectron spectroscopy and X-ray absorption. The results show that the doping atom influences the level of stoichiometry, as well as the induced disorder and the change in the partial ionic character of the Y-O bond.     

Metastable de-excitation spectroscopy and UV photoelectron spectroscopy study of polyacrylonitrile layers

An electropolymerized polyacrylonitrile layer (200 nm thick) was studied by metastable de-excitation spectroscopy (He 2 ³S) and UV photoelectron spectroscopy (HeI). We show that this layer has both insulating and conducting properties. Its electronic properties and its dielectric behaviour change appreciably after reticulation and with temperature.The importance of the nitrile group in the understanding of these properties is emphasized.     

Elementary Mechanisms in the Interaction of Organic Molecules with Mineral Surfaces

Model systems are used to reproduce the actual situations encountered during the interaction of organic molecules with mineral solids under controlled experimental conditions.

The role of local electrical fields produced by surface heterogeneities and the influence of the electronic properties of the functional groups of the organic molecule and of the surface are demonstrated and discussed.

This study enables modelling of the basic mechanisms in the organic molecule-mineral solid bond and of the polymerization process of the organic film.

The results given here may be used to produce theoretical models of predictive nature in the field of adhesion and structural bonding.     

Behaviour of α-AI2O3 insulator surfaces under electron irradiation

Cathodoluminescence (CL), Auger electron spectroscopy (AES) and direct crack measurements were performed on-Al₂O₃ samples in order to relate chemical, electrical and mechanical effects induced by electron irradiation of the surface. Electrical discharges and visible luminescence were observed during the excitation of the samples with a 80 keV electron beam. Changes of the surface state and of the toughnessK _Ic were subsequently detected. The results suggest that charging of the sample is related to the presence of defects and corresponding trap levels in the energy gap. The concentration of defects (oxygen vacancies or associated F, F⁺ centres) may be enhanced, especially in the vicinity of the surface, by the electrical discharges induced by the electron irradiation. This leads to an increase of mechanical stresses in the brittle material: a striking example was the fracture of a corundum single crystal in the electron microscope, which cannot be explained by the direct heating effect of the primary electron beam. On the contrary, an advantageous situation for the mechanical properties of the material may be achieved when the defects have a blocking effect on the crack propagation; a subsequent increase of the toughnessK _Ic is then recorded.